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3 | RESULTS AND DISCUSSION
the pH became neutral. The schematic diagram in Figure 1 illustrates how the application of nitric acid steam modifies the pine wood fiber, providing a visual rep- resentation of the introduced nitro groups and the result- ing structural changes in each component. Additionally, the different reaction times were tested to investigate the increment in the carboxylic acid content and change in the chemical and morphological structure. The carboxylation process involves the introduction of carboxylic acid groups onto the wood fibers, as con- firmed by the FTIR (see Figure 2). The changes in the peak intensity of the carbonyl group of carboxylic acids, formed by testing different reaction times, were observed around 1720 cm 1 . The absence of the carbonyl group at 1720 cm 1 in the raw fibers, compared with the modified fibers, is due to the C O bond in lignin being formed and subsequently cleaved during the carboxylation pro- cess, which confirms the successful carboxylation of the fibers. Additionally, the peak around 1507 cm 1 is associ- ated with the aromatic C O of lignin. 35,36 The peak at 1507 cm 1 is often associated with the aromatic ring stretching vibrations (C C) in lignin. When the wood fibers are treated with HNO 3 gas, the oxidation process leads to the modification of lignin mainly. As a result, the intensity of this peak is expected to decrease and shift due to the alteration of the lignin structure. The peak at
3.1 | Investigation of unmodified wood fibers
The structural and morphological properties of unmodi- fied fibers were investigated using an optical microscope (Figure S1). Raw fibers analyzed with Image J software had lengths of 4.042 ± 0.70 mm and thicknesses of 0.1368 ± 0.673 mm. After modifications for 15, 30, and 60 min, average sizes were 3.17 ± 0.68 mm (thickness 0.0686 ± 0.0035 mm), 1.70 ± 0.30 mm (thickness 0.0397 ± 0.0044 mm), and 0.47 ± 0.15 mm (thickness 0.0243 ± 0.0062 mm), respectively. The monotonic reduction in fiber size is due to the partial disintegration of fiber bun- dles during modification. Notably, the 60-min modifica- tion showed significant size reduction with a more uniform size distribution compared with shorter dura- tions. Also, raw fibers have lower aspect ratios than those treated for 15 and 30 min, while the 60-min treatment showed the lowest aspect ratio. Optimal aspect ratios are crucial for maximizing fiber reinforcement, particularly in strong fiber-matrix bonds and lower fiber strength, 34 therefore, the fibers modified for 15 and 30 min, with their higher aspect ratios, can provide better reinforce- ment for wood panels, whereas 60 min modification deteriorates.
3.2 | Carboxylation of the pine wood fibers
First, the effect of the treatment time with nitric acid (65 wt%) steam was studied by treating 20 g of fibers for 15, 30, and 60 min at a temperature of 130 C (see experi- mental section and Figure 1 for details). This reaction occurs after the thermal decomposition of nitric acid at higher temperatures. The nitric acid functionalization process is primarily based on the thermal decomposition of nitric acid. The reaction proceeds via a simple oxida- tive process utilizing nitric acid, specifically applied at its decomposition temperature (130 C) producing NO 2 gas. Under nitro-oxidation conditions, the reaction is initiated as nitro species (e.g., NO 2 or NO 3 ) can attack the carbon atom of the primary alcohol on the surface of the wood fiber, forming a positively charged intermediate carbon atom. In the second step, the oxygen atom on the nitro- gen dioxide attacks the carbon atom in the same interme- diate, resulting in the carboxylic acid group. Subsequent substitution reactions and rearrangements, coupled with the formation of oxidation, introduce carbonyl or carbox- ylic acid groups into the lignin structure. After that, the excess acid was washed with deionized water until
FIGURE 2 (A) Fourier-transform infrared spectroscopy (FTIR) data of raw, 15, 30, and 60 min CA'ed modified fiber and (B) the same spectra in the wavenumber range of 1200 – 2000 cm 1 , respectively. (C) X-ray diffraction (XRD) data of raw fiber, 15 min modified fiber, 30 min modified fiber, and 60 min modified fiber and (D) UV – Vis reflectance results for raw, 15, 30, and 60 min CA'ed modified fiber.
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